Nanotechnology comprises technological developments within the nanometer scale (between 1 and 100 nm). Interdisciplinary, it falls within the field of nanoscience and turned to its advances, which involves physics, chemistry, engineering, computer science, biology and environmental sciences. Governed by the laws of quantum physics, the universe of nanoparticles may cause materials to present different behaviors in the nanoscale than shown in the normal macro and microscopic scale. For example, some materials that are naturally opaque or translucent to the naked eye exhibit transparent behavior at the nanoscale. Some materials are known as “smart materials”, since their new properties under the treatment of nanotechnology can be combined with their original attributes, to play multiple or adaptive functions, such as by enabling the interfacing of organic and inorganic materials (Leite, E. R. Nanocrystals assembled from the bottom up. In: NALWA, H. S. (Ed.). Encyclopedia of nanoscience and nanotechnology. Washington: American Scientific Publishers, 2004. v. 6, p. 537-554).
Nanomaterials by definition have at least one dimension in the nanometer scale. Currently being added to polymers, nanomaterials aim at stimulating changes in the physical, chemical and mechanical polymeric behaviors, or to confer entirely new properties to polymers (M. Alexandre, P. Dubois Materials Science and Engineering 28 (2000) 1±63; S. Sinha Ray, M. Okamoto Prog. Polym. Sci. 28 (2003) 1539-1641). As a practical matter, it can be highlighted that the use of antimicrobial additives to induce bactericidal and/or fungicidal properties to polymers can promote:                Total or partial elimination of the harmful effects of microbes in healthcare, including prevention of new, virulent mutations that may pose health risks and/or reduce the longevity of the products;        Effective sterilization, promoting cleaner products and environments, helping to prevent diseases and supporting preventive health and wellness. Sterilization also avoids the formation of mold and other causes of unpleasant odors, improving surface cleanliness and reduction of the maintenance costs;        Improvement of strategies used by the food industry to sustain or increase product shelf life; examples are enhanced antimicrobial containers that keep the original food flavor, leading to healthier products that also preserve the original food properties.        
Types of Antimicrobial Additives
Antimicrobial additives may be classified as natural or chemical. Natural additives are inorganic, silver-based compounds, such as a core of oxides, halides and glass particles (silica, titania, zeolite), coated with silver-based compounds. Chemical additives, on the other hand, are based on compounds such as chlorophenol and thiabendazole (Frost & Sullivan. Strategic Analysis of the European Plastic Packaging Additives Market M193-39. April 2009).
There is a current trend towards the use of natural silver-based antimicrobial additives over the, chemical ones, mainly for two reasons:                Organic chemical additives represent a possible health threat;        The development of resistance in bacteria seems more closely related to organic chemical additives than to natural ones.        
Organic antimicrobial chemical additives are more prone to leaching than silver-based additives, leading to a reduction of its effectiveness or even efficacy (functionality). There is a concern that if chemical additives are used in applications such as containers for food packaging, they may migrate into the food. Chlorophenol, for example, has been detected in breast milk after use in food packaging, also raising concerns about a possible role of this chemical additive in the development of cancer.
Natural silver-based compounds have the major advantage of being static, generally not subjected to leaching, with a solid and lasting effect throughout the product's life. In the unlikely event that a silver-based additive does migrate from the container to the surface of the product, they should not represent a significant health risk, since they are nontoxic, inert and non-corrosive.
Throughout the ages, silver has been used for applications where a sterile environment is required. It is known to be safe, odorless, and highly effective. As a result, the silver-based are the most used antimicrobial additives by the plastic packaging industry. Unlike organic additives, silver is also rarely associated with the onset of resistance in microorganisms.
On the manufacturing side, silver-based antimicrobials display heat stability, being stable up to about 800° C. They can be processed at high temperatures and incorporated into polymers prior to processing, even at high processing temperature. Organic antimicrobials may decompose under high temperatures.
Natural silver-based compounds maintain its bactericidal efficacy in aqueous media, leading to a higher longevity (service life) than organic additives. As a declaration of its non-toxicity and high efficacy, silver-based antimicrobial additives are the additives of choice by healthcare organizations. Organic additives have also been under observation because of safety concerns, and impending regulations may restrict their use in the future, especially in uses like food packaging.
Historically, silver has been used as an effective treatment, against hundreds of infectious conditions, due to the selective effect of the silver ions on microorganisms (bacteria, fungi, viruses). This effect is through at least three distinct mechanisms: (1) transport functions in the cell wall (respiration); (2) cell division (reproduction); and (3) generation of energy (metabolism). The negative effect of silver on these vital functions leads to a growth inhibition in microorganisms, the so-called bacteriostatic effect. For bacteria, it also prevents the colonization of other, different but related bacterial strains.
Silver-based compounds can either be (1) attached directly to a surface, by complexing them with a given polymer, or (2) anchored to a surface by prior impregnation to a ceramic particle (silica, zeolite, titania), before incorporation into the polymer.
Several methods have been developed to prepare silver nanoparticles, including chemical reduction, laser ablation and photoreduction, but chemical reactions in solution (wet chemistry) is the most commonly used. Classic wet chemistry methods typically start by first mixing metal salts, which are then reduced by a chemical agent to produce colloidal suspensions which then result in metal nanoparticles [6]. Sodium borohydride and trisodium citrate have been widely used as reducing agents. Both are proven to produce monodisperse colloids with size and desired characteristics (M. V. Canamares, J. V. Garcia-Ramos, J. D. Gomez-Varga, C. Domingo, S. Sanches-Cortes, Langmuir, 21, 2005, pp. 8546-8553; R. S. Sheng, L. Zhu, M. D. Morris, Anal. Chem., 58, 1986, pp. 1116-1119).
Nanotechnology-based antimicrobial additives have a superior bacteriostatic effect, since their high surface makes up for a relatively large contact area between their active component, silver, and the microorganism surface. This allows lower concentrations of antimicrobial nanoadditives to be used, as compared to additives in higher scales, such as the micrometer scale.